1. + Chapter 2: Modeling Distributions of Data Section 2.1 Describing Location in a Distribution The Practice of Statistics, 4 th edition - For AP* STARNES, YATES, MOORE

2. + Describing Location in a Distribution Measuring Position: Percentiles One way to describe the location of a value in a distributionis to tell what percent of observations are less than it. Definition: The p th percentile of a distribution is the value with p percent of the observations less than it. 6 7 7 2334 7 5777899 8 00123334 8 569 9 03 Jenny earned a score of 86 on her test. How did she perform relative to the rest of the class? Example, p. 85 Her score was greater than 21 of the 25 observations. Since 21 of the 25, or 84%, of the scores are below hers, Jenny is at the 84 th percentile in the class’s test score distribution. 6 7 7 2334 7 5777899 8 00123334 8 569 9 03

3. + Cumulative Relative Frequency Graphs A cumulative relative frequency graph (or ogive ) displays the cumulative relative frequency of eachclass of a frequency distribution. Describing Location in a Distribution Age of First 44 Presidents When They Were Inaugurated AgeFrequencyRelative frequency Cumulative frequency Cumulative relative frequency 40- 44 22/44 = 4.5% 22/44 = 4.5% 45- 49 77/44 = 15.9% 99/44 = 20.5% 50- 54 1313/44 = 29.5% 2222/44 = 50.0% 55- 59 1212/44 = 34% 3434/44 = 77.3% 60- 64 77/44 = 15.9% 4141/44 = 93.2% 65- 69 33/44 = 6.8% 4444/44 = 100%

4. + Describing Location in a Distribution Use the graph from page 88 to answer the following questions. Was Barack Obama, who was inaugurated at age 47,unusually young? Estimate and interpret the 65 th percentile of the distribution Interpreting Cumulative Relative Frequency Graphs 47 11 65 58

5. + Describing Location in a Distribution Measuring Position: z -Scores A z -score tells us how many standard deviations from the mean an observation falls, and in what direction. Definition: If x is an observation from a distribution that has known mean and standard deviation, the standardized value of x is: A standardized value is often called a z-score. Jenny earned a score of 86 on her test. The class mean is 80 and the standard deviation is 6.07. What is her standardized score?

6. + Describing Location in a Distribution Using z -scores for Comparison We can use z-scores to compare the position of individuals in different distributions. Jenny earned a score of 86 on her statistics test. The class mean was 80 and the standard deviation was 6.07. She earned a score of 82 on her chemistry test. The chemistry scores had a fairly symmetric distribution with a mean 76 and standard deviation of 4. On which test did Jenny perform better relative to the rest of her class? Example, p. 91

7. + Example, p. 93 Describing Location in a Distribution Transforming Data Transforming converts the original observations from the original units of measurements to another scale. Transformations can affect the shape, center, and spread of a distribution. Adding the same number a (either positive, zero, or negative) to each observation: adds a to measures of center and location (mean, median, quartiles, percentiles), but Does not change the shape of the distribution or measures of spread (range, IQR, standard deviation). Effect of Adding (or Subtracting) a Constant nMeansxsx MinQ1Q1 MQ3Q3 MaxIQRRange Guess(m)4416.027.14811151740632 Error (m)443.027.14-5-22427632

8. + Example, p. 95 Describing Location in a Distribution Transforming Data Multiplying (or dividing) each observation by the same number b (positive, negative, or zero): multiplies (divides) measures of center and location by b multiplies (divides) measures of spread by |b|, but does not change the shape of the distribution Effect of Multiplying (or Dividing) by a Constant nMeansxsx MinQ1Q1 MQ3Q3 MaxIQRRange Error(ft)449.9123.43-16.4-6.566.5613.1288.5619.68104.96 Error (m)443.027.14-5-22427632